1. Technical Field of the Invention
The present invention relates to drain structures provided in casings of electric rotating machines.
2. Description of the Related Art
There are known electric rotating machines which have drain structures for draining water, which has intruded into a casing of the machine, from the inside to the outside of the casing (see, for example, Japanese Patent Application Publication No. H04-150752). In addition, electric rotating machines include, for example, electric motors and electric generators.
FIG. 9A shows a conventional drain structure of a starter for starting an engine of a motor vehicle. As shown in the figure, the starter includes a casing 101 that has a through-hole 102 formed therein. To the through-hole 102, there is mounted a drain member 103.
Specifically, the drain member 103 includes a mounting portion 104 that extends in the penetration direction of the through-hole 102 of the casing 101. The mounting portion 104 has a pair of hooks 105 that are arranged so as to face each other in a direction perpendicular to the penetration direction of the through-hole 102. Each of the hooks 105 includes a proximal part 108 and a distal part 109. The proximal part 108 extends in the penetration direction of the through-hole 102. The distal part 109 has a distal end (or tip) 111 that is positioned furthest from the proximal part 108 and makes up an insertion end 111 of the mounting portion 104.
The mounting portion 104 of the drain member 103 is mounted to the through-hole 102 of the casing 101 by inserting the hooks 105 into the through-hole 102 from the outside of the casing 101. More specifically, during the insertion of the hooks 105 into the through-hole 102, the hooks 105 first flex so as to reduce the distance between them. After the distal parts 109 of the hooks 105 has passed through the through-hole 102, the hooks 105 return to their original shapes, and the distal parts 109 are caught on the inner surface of the casing 101 around the through-hole 102, as illustrated with two-dot chain lines in FIG. 9A.
Moreover, in the conventional drain structure, there is provided a chamfer 112 on the outer surface of the casing 101 around the through-hole 102. With the chamfer 112, the entrance of the through-hole 102 is widened with respect to the distance between the insertion ends 111 of the mounting portion 104 of the drain member 103. Consequently, even if the insertion ends 111 are deviated from their desired positions in the flexural direction of the hooks 104, it is still possible to insert the hooks 104 into the through-hole 102. That is, with the chamfer 112, the allowable amount z of positional deviation of the insertion ends 111 with respect to the through-hole 102 before the insertion of the hooks 105 into the through-hole 102 is increased.
Furthermore, to meet the recent cost-saving requirement, it is necessary to minimize the thickness of the casing 101. However, with the minimization of the thickness of the casing 101, it may become impossible to provide the chamfer 112 around the through-hole 102. Further, without the chamfer 112, the allowable amount z of positional deviation of the insertion ends 111 with respect to the through-hole 102 may be reduced. As a result, it may become necessary to accordingly improve the accuracy of mounting the drain member 103 to the through-hole 102, thereby increasing the manufacturing cost of the starter.
Therefore, it is desired to set large the allowable amount z of positional deviation of the insertion ends 111 with respect to the through-hole 102 without providing the chamfer 112.
To meet the above desire, one may consider applying an alternative drain structure as shown in FIG. 9B. In this drain structure, the distance between the facing pair of the hooks 105 is reduced, thereby setting the distance s between the insertion ends 111 sufficiently smaller than the width d of the through-hole 102 on the outer surface of the casing 101.
However, with the above alternative drain structure, the distances between the proximal parts 108 of the hooks 105 and the inner wall of the through-hole 102 (i.e., the inner wall of the casing 101 which defines the through-hole 102) after the mounting of the mounting portion 104 of the drain member 103 to the through-hole 102 are increased. Thus, the allowable amount k of movement of the hooks 105 within the through-hole 102 after the mounting of the mounting portion 104 to the through-hole 102 is accordingly increased. Consequently, with the increase in the allowable amount k, it becomes easier for the hooks 105 to move within the through-hole 102 due to vibrations caused by, for example, operation of the starter or an external force. As a result, in the worst case, the distal parts 109 of the hooks 105, which are normally caught on the inner surface of the casing 101, will be detached from the inner surface.
In addition, if the thickness of the proximal parts 108 of the hooks 105 in the flexural direction of the hooks 105 is set too large, the hooks 105 may be easily damaged by stress concentration during the flexing of the hooks 105. Therefore, it is also desired to set small the thickness of the proximal parts 108 of the hooks 105 in the flexural direction of the hooks 105.